1. Technical Field
The present invention relates to a semiconductor device, and more particular, to a III-V nitride semiconductor device, and also relates to a protection element comprised of such a semiconductor device and used to protect a switching element of a power conversion apparatus, and to a power conversion apparatus including the protection element.
2. Related Art
A variety of electronic devices constituted by semiconductor devices are known, such as for example, a switching element for a power conversion apparatus that is constituted by a bipolar transistor having a high withstand voltage. Such a switching element for high power application is required to have a high withstand voltage and a low on-resistance. To this end, instead of a bipolar transistor, a power metal oxide semiconductor FET (power MOSFET) having a low on-resistance or an insulated gate bipolar transistor (IGBT) which is a combination of a bipolar transistor and an MOSFET has been recently employed as a switching element.
On the other hand, a power MOSFET entails a parastic bipolar transistor effect, and an FET element can be destroyed due to an inrush current or surge voltage that is applied to the MOSFET upon start of operation. Thus, a protection element for preventing a parastic effect and FET element destruction is required. For example, a zener diode of pn junction structure has been employed for protection of an Si-based MOSFET.
With a zener diode of pn junction structure, however, it is difficult to satisfactorily protect a switching element that is constituted by an MOSFET having a low on-voltage for the following reasons. That is, a zener diode of pn junction structure, having a low withstand voltage of about 100 volts, a high on-resistance of about 10 mxcexa9cm2 and a high on-voltage of about 1.2 volts to 1.5 volts, cannot sufficiently withstand an inrush current or surge voltage applied thereto upon start of operation of an MOSFET, can generate heat upon application of a surge voltage, and can permit an MOSFET having a low on-voltage to be destroyed before the zener diode starts to make a protecting action. Therefore, it is difficult to properly protect a switching element comprised of an MOSFET with use of a zener diode of pn junction structure, so as to ensure an operational reliability of a power conversion apparatus provided with such a switching element. Moreover, in a case where a zener diode of pn junction structure is utilized for protection of an MOSFET, a high loss can be caused to lower the efficiency of a power conversion apparatus.
It is known that a III-V nitride semiconductor device has a high withstand voltage and a low on-resistance. Thus, a further improvement of such advantages and concrete applications to electronic devices utilizing such advantages have been demanded.
An object of the present invention is to provide a III-V nitride semiconductor device having a high withstand voltage and a low on-voltage, such as a semiconductor Schottky diode and a semiconductor Schottky gate field effect transistor.
Another object of the present invention is to provide a protection element that is used for protection of a switching element for power conversion and that is constituted by a III-V nitride semiconductor device.
A further object of the present invention is to provide a power conversion apparatus that comprises a protection element constituted by a III-V nitride semiconductor device and that is highly efficiently operable with high reliability.
According to one aspect of the present invention, a GaN-based semiconductor device as a III-V nitride semiconductor device is provided. The GaN-based semiconductor device comprises: a III-V nitride semiconductor layer having a surface portion thereof shaped to form a protrusion; a first anode electrode forming a Schottky junction with an upper face of the protrusion of the III-V nitride semiconductor layer; and a second anode electrode forming a Schottky junction with a side face of the protrusion of the III-V nitride semiconductor layer and to be electrically connected with the first anode electrode, wherein a Schottky barrier formed between the first anode electrode and the III-V nitride semiconductor layer is lower in height than that formed between the second anode electrode and the III-V nitride semiconductor layer, whereby a GaN-based semiconductor device, e.g., a semiconductor Schottky diode, having a low on-voltage and high withstand voltage can be realized.
According to another aspect of the present invention, a GaN-based semiconductor device is provided, which comprises: a III-V nitride semiconductor layer having a predetermined width; a first anode electrode forming a Schottky junction with the III-V nitride semiconductor layer, with a junction width narrower than the predetermined width of the III-V nitride semiconductor layer; and a second anode electrode forming a Schottky junction with a portion of the III-V nitride semiconductor layer other than a portion at which the first anode electrode is in contact with the III-V nitride semiconductor layer, the second anode electrode being electrically connected with the first anode electrode, wherein a Schottky barrier formed between the first anode electrode and the III-V nitride semiconductor layer is lower in height than that formed between the second anode electrode and the III-V nitride semiconductor layer, whereby a semiconductor Schottky diode having a low on-voltage and high withstand voltage can be realized, as in the first-mentioned aspect of this invention.
According to still another aspect of the present invention, a III-V nitride semiconductor device is provided, which comprises: an electrically conductive substrate; a III-V nitride semiconductor layer formed on the substrate and having a surface portion thereof shaped to form a protrusion; a source electrode formed on an upper face of the protrusion of the III-V nitride semiconductor layer so as to form an ohmic junction therewith; a gate electrode formed on a side face of the protrusion so as to form a Schottky junction therewith; and a drain electrode formed on a rear face of the substrate so as to form an ohmic junction therewith, whereby a vertical semiconductor Schottky gate field effect transistor constituted by a III-V nitride semiconductor can be realized.
According to a further aspect of the present invention, there is provided a protection element for protecting a switching element constituting a power conversion circuit of a power conversion apparatus. The protection element is a GaN-based Schottky diode or GaN-based field effect transistor that has an on-voltage not higher than 1 volt and a withstand voltage not lower than 300 volts, whereby the switching element is permitted to easily operate with a low on-voltage to thereby contribute to higher efficiency of the power conversion apparatus, and the switching element is protected by an inrush current or a surge voltage applied thereto upon start of operation, thereby achieving an improved voltage withstand property and high-current operation of the switching element and improving reliability of the power conversion apparatus.
According to a further aspect of the present invention, a power conversion apparatus is provided, which comprises a power conversion circuit including a switching element; and a protection element, constituted by a GaN-based Schottky diode or a GaN-based field effect transistor, for protecting the switching element, whereby the power conversion apparatus has improved reliability and higher efficiency.